High voltage output driver circuits are useful for driving a variety of electrical loads such as wire dot print heads, micropumps of ink-jet printers, optical switches, piezoelectric relays, and the like. One particular type of piezoelectric relay utilizes a bimorph element.
Bimorph elements typically consist of two plates of piezoelectric material sandwiched between three planar electrodes. The piezoelectric material is typically a ceramic such as lead zirconate titanate. The first electrode is located on the outer surface of the first plate. The second electrode is sandwiched between the two plates. Finally, the third electrode is located on the outer surface of the second plate. When a high voltage is applied across one of the plates, the plate changes length. If, for example, only one of the two plates is subjected to such a voltage, the bimorph will bend in a direction perpendicular to the two plates.
This bending effect is used to construct relays. In a piezoelectric relay, one end of the bimorph is mounted in a cantilever manner over a surface. The bending motion of the bimorph is used to make or break a connection between two contacts. The first contact is mounted on the free end of the bimorph. The second contact is mounted on the surface. When a voltage is applied across the appropriate plate in the bimorph, the free end of the bimorph causes the contact mounted thereon to move with respect to the contact mounted on the surface. By applying a voltage across one of the two plates, the contacts can be made to move such that they are forced together, thus, completing an electrical circuit. When a voltage is applied across the other plate, the contacts move apart, thus breaking the circuit in question. In a double pole relay, this breaking motion can be used to complete a second circuit by forcing a second pair of contacts together. This second pair of contacts consists of a contact on the bimorph and a contact mounted on a second surface.
Piezoelectric relays of this type are particularly well suited to telecommunications applications. In particular, such relays provide an economical means for constructing cross-connect switches. In many telecommunication applications, switches are utilized which may contain thousands relays. If these relays are piezoelectric relays, the appropriate driving voltage must be maintained on each of these relays at all times. Eac relay requires a separate driving circuit which must be capable of supplying voltages of the order of a few hundreds of volts. Hence, it is very important that the power dissipation of each individual driving circuit be kept at a minimum.
Since each piezoelectric strip in a bimorph is electrically equivalent to a capacitor, the power dissipated by each driving circuit consists of the power needed to change the state of the relay, i.e., to charge and discharge the capacitors, and the power dissipated in the driving circuit itself in maintaining the voltage across each of the capacitors. This later power dissipation will be referred to hereinafter as the quiescent power dissipation. In practice, the state of each relay is changed very infrequently; hence substantially all of the power dissipation is associated with the quiescent power dissipation.
In addition to having low power dissipation, it is important that the driving circuit be capable of fabrication using integrated circuit technology. In many applications of interest, a large number of relays must be driven simultaneously. Therefore, a large number of driving circuits must be supplied. It would be advantageous to able to construct all of the driving circuits in question on a single integrated circuit chip. It is important, therefore, that the number of transistors needed for each driving circuit be minimized. Since high voltage transistors require a substantial silicon surface area on the integrated circuit chip, it is particularly important that the number of high voltage transistors be minimized.
Broadly, it is an object of the present invention to provide an improved high voltage driving circuit for switching a circuit element between two voltages.
It is a further object of the present invention to provide a high voltage driving circuit with minimal quiescent power dissipation.
It is yet another object of the present invention to provide a high voltage driving circuit that can be fabricated using a minimum number of high voltage transistors.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the present invention and the accompanying drawings.